The main date and time API is built on the ISO calendar system.
The chronology operates behind the scenes to represent the general concept of a calendar system.
For example, the Japanese, Minguo, Thai Buddhist and others.

Most other calendar systems also operate on the shared concepts of year, month and day,
linked to the cycles of the Earth around the Sun, and the Moon around the Earth.
These shared concepts are defined by ChronoField and are available
for use by any Chronology implementation:

As shown, although the date objects are in different calendar systems, represented by different
Chronology instances, both can be queried using the same constant on ChronoField.
For a full discussion of the implications of this, see ChronoLocalDate.
In general, the advice is to use the known ISO-based LocalDate, rather than
ChronoLocalDate.

While a Chronology object typically uses ChronoField and is based on
an era, year-of-era, month-of-year, day-of-month model of a date, this is not required.
A Chronology instance may represent a totally different kind of calendar system,
such as the Mayan.

In practical terms, the Chronology instance also acts as a factory.
The of(String) method allows an instance to be looked up by identifier,
while the ofLocale(Locale) method allows lookup by locale.

The Chronology instance provides a set of methods to create ChronoLocalDate instances.
The date classes are used to manipulate specific dates.

Adding New Calendars

The set of available chronologies can be extended by applications.
Adding a new calendar system requires the writing of an implementation of
Chronology, ChronoLocalDate and Era.
The majority of the logic specific to the calendar system will be in the
ChronoLocalDate implementation.
The Chronology implementation acts as a factory.

To permit the discovery of additional chronologies, the ServiceLoader
is used. A file must be added to the META-INF/services directory with the
name 'java.time.chrono.Chronology' listing the implementation classes.
See the ServiceLoader for more details on service loading.
For lookup by id or calendarType, the system provided calendars are found
first followed by application provided calendars.

Each chronology must define a chronology ID that is unique within the system.
If the chronology represents a calendar system defined by the
CLDR specification then the calendar type is the concatenation of the
CLDR type and, if applicable, the CLDR variant,

Implementation Requirements:

This interface must be implemented with care to ensure other classes operate correctly.
All implementations that can be instantiated must be final, immutable and thread-safe.
Subclasses should be Serializable wherever possible.

ofLocale

This returns a Chronology based on the specified locale,
typically returning IsoChronology. Other calendar systems
are only returned if they are explicitly selected within the locale.

The Locale class provide access to a range of information useful
for localizing an application. This includes the language and region,
such as "en-GB" for English as used in Great Britain.

The Locale class also supports an extension mechanism that
can be used to identify a calendar system. The mechanism is a form
of key-value pairs, where the calendar system has the key "ca".
For example, the locale "en-JP-u-ca-japanese" represents the English
language as used in Japan with the Japanese calendar system.

This method finds the desired calendar system by in a manner equivalent
to passing "ca" to Locale.getUnicodeLocaleType(String).
If the "ca" key is not present, then IsoChronology is returned.

Note that the behavior of this method differs from the older
Calendar.getInstance(Locale) method.
If that method receives a locale of "th_TH" it will return BuddhistCalendar.
By contrast, this method will return IsoChronology.
Passing the locale "th-TH-u-ca-buddhist" into either method will
result in the Thai Buddhist calendar system and is therefore the
recommended approach going forward for Thai calendar system localization.

A similar, but simpler, situation occurs for the Japanese calendar system.
The locale "jp_JP_JP" has previously been used to access the calendar.
However, unlike the Thai locale, "ja_JP_JP" is automatically converted by
Locale to the modern and recommended form of "ja-JP-u-ca-japanese".
Thus, there is no difference in behavior between this method and
Calendar#getInstance(Locale).

of

Obtains an instance of Chronology from a chronology ID or
calendar system type.

This returns a chronology based on either the ID or the type.
The chronology ID uniquely identifies the chronology.
The calendar system type is defined by the
CLDR specification.

The chronology may be a system chronology or a chronology
provided by the application via ServiceLoader configuration.

Since some calendars can be customized, the ID or type typically refers
to the default customization. For example, the Gregorian calendar can have multiple
cutover dates from the Julian, but the lookup only provides the default cutover date.

getCalendarType

The calendar type is an identifier defined by the CLDR and
Unicode Locale Data Markup Language (LDML) specifications
to uniquely identification a calendar.
The getCalendarType is the concatenation of the CLDR calendar type
and the variant, if applicable, is appended separated by "-".
The calendar type is used to lookup the Chronology using of(String).

Returns:

the calendar system type, null if the calendar is not defined by CLDR/LDML

dateNow

Obtains the current local date in this chronology from the specified clock.

This will query the specified clock to obtain the current date - today.
Using this method allows the use of an alternate clock for testing.
The alternate clock may be introduced using dependency injection.

date

This obtains a date in this chronology based on the specified temporal.
A TemporalAccessor represents an arbitrary set of date and time information,
which this factory converts to an instance of ChronoLocalDate.

The conversion typically uses the EPOCH_DAY
field, which is standardized across calendar systems.

This method matches the signature of the functional interface TemporalQuery
allowing it to be used as a query via method reference, aChronology::date.

localDateTime

Obtains a local date-time in this chronology from another temporal object.

This obtains a date-time in this chronology based on the specified temporal.
A TemporalAccessor represents an arbitrary set of date and time information,
which this factory converts to an instance of ChronoLocalDateTime.

The conversion extracts and combines the ChronoLocalDate and the
LocalTime from the temporal object.
Implementations are permitted to perform optimizations such as accessing
those fields that are equivalent to the relevant objects.
The result uses this chronology.

This method matches the signature of the functional interface TemporalQuery
allowing it to be used as a query via method reference, aChronology::localDateTime.

zonedDateTime

Obtains a ChronoZonedDateTime in this chronology from another temporal object.

This obtains a zoned date-time in this chronology based on the specified temporal.
A TemporalAccessor represents an arbitrary set of date and time information,
which this factory converts to an instance of ChronoZonedDateTime.

The conversion will first obtain a ZoneId from the temporal object,
falling back to a ZoneOffset if necessary. It will then try to obtain
an Instant, falling back to a ChronoLocalDateTime if necessary.
The result will be either the combination of ZoneId or ZoneOffset
with Instant or ChronoLocalDateTime.
Implementations are permitted to perform optimizations such as accessing
those fields that are equivalent to the relevant objects.
The result uses this chronology.

This method matches the signature of the functional interface TemporalQuery
allowing it to be used as a query via method reference, aChronology::zonedDateTime.

eraOf

The era is, conceptually, the largest division of the time-line.
Most calendar systems have a single epoch dividing the time-line into two eras.
However, some have multiple eras, such as one for the reign of each leader.
The exact meaning is determined by the chronology according to the following constraints.

The era in use at 1970-01-01 must have the value 1.
Later eras must have sequentially higher values.
Earlier eras must have sequentially lower values.
Each chronology must refer to an enum or similar singleton to provide the era values.

This method returns the singleton era of the correct type for the specified era value.

range

All fields can be expressed as a long integer.
This method returns an object that describes the valid range for that value.

Note that the result only describes the minimum and maximum valid values
and it is important not to read too much into them. For example, there
could be values within the range that are invalid for the field.

This method will return a result whether or not the chronology supports the field.

resolveDate

Most TemporalField implementations are resolved using the
resolve method on the field. By contrast, the ChronoField class
defines fields that only have meaning relative to the chronology.
As such, ChronoField date fields are resolved here in the
context of a specific chronology.

The default implementation, which explains typical resolve behaviour,
is provided in AbstractChronology.

Parameters:

fieldValues - the map of fields to values, which can be updated, not null

resolverStyle - the requested type of resolve, not null

Returns:

the resolved date, null if insufficient information to create a date

Throws:

DateTimeException - if the date cannot be resolved, typically
because of a conflict in the input data

period

This returns a period tied to this chronology using the specified
years, months and days. All supplied chronologies use periods
based on years, months and days, however the ChronoPeriod API
allows the period to be represented using other units.

Implementation Requirements:

The default implementation returns an implementation class suitable
for most calendar systems. It is based solely on the three units.
Normalization, addition and subtraction derive the number of months
in a year from the range(ChronoField). If the number of
months within a year is fixed, then the calculation approach for
addition, subtraction and normalization is slightly different.

If implementing an unusual calendar system that is not based on
years, months and days, or where you want direct control, then
the ChronoPeriod interface must be directly implemented.